Polishing of metal surfaces



Patented Apr. 6, 1943 consume or METAL SURFACES Charles L. Faust, Columbus, Ohio,

asslgnor to Battelle Memorial Institute, Columbus, Ohio, a corporation of Ohio No Drawing. Application August 16, 1939, Serial N0. 290,493

2 Claims. (Cl. 204-140) This invention relates to an electrolyte for the polishing of metal surfaces and more particularly to an electrolytic solution of an improved composition for use in the anodic treatment of metals to impart thereto a lustrous, mirror-like polish.

I have now found that the presence of perchloric acid in various types of electrolytic solutions used for the electro-polishing of metals produces a considerable improvement in the de gree of polish or luster obtained. Improved polishes are obtained on stainless steels, either straight chromium stainless steels or chromiumnickel stainless steels, nickel metals and nickel alloys, such as Monel metal, German silver and chromel, and plain carbon steels.

It is therefore an important object of this invention to provide an electrolytic solution containing perchloric acid for use in the anodic treatment of metals to impart thereto a highly lustrous surface having characteristics that are relatively superior to those obtainable by mechanical polishing or bufling operations.

Other and further important objects of this invention will become apparent from the following description and appended claims.

The method to which this invention pertains involves making the metal to be polished an anode in an electrolytic bath of suitable composition and passing a current of sufficient density and for a suillcient length of time to produce a high luster or polish on the metal. By employing the electrolytic solution of my present invention, results are obtained that transcend a mere electrolytic cleaning process in that there is produced a highly lustrous or polished surface. This high polish or luster is an important feature of my invention and one that distinguishes it sharply from prior art finishes produced in the electrolytic cleaning of metals.

The electrolytic solution that I have now found to possess a quite general applicability in the electropolishing of ferrous metals, chromium and chrome-nickel steels, nickel and nickel alloys, contains perchloric acid or compounds furnishing the perchlorate radical in effective amounts as an active ingredient of the solution. A preferred type of solution is one containing sulfuric acid, ortho phosphoric acid, and perchloric acid, with the balance substantially water.

While it is possible to produce polishes with aqueous mixtures of sulfuric, phosphoric and perchloric acids, in which the water content is as high as 50%, I prefer to keep the water content of the electrolytic bath relatively low, since I have found, in general, that baths containing lower proportions of water may be operated at lower current densities, and therefore at a, lower cost, to give satisfactory polishes. I therefore prefer, in the aqueous type of bath, to have the total acid content in excess of 50% by weight. The relative proportion of sulfuric acid may be varied from 5% to of phosphoric acid from 5% to 85%, and of perchloric acid (or equivalent perchlorate radical) from 0.1% up to 30%, but in any case the total acid concentration should be at least 50% of the resulting solution.

Unless otherwise specified, all percentages referred to herein will be understood to represent percentages by weight, rather than by volume. Also, where the acids are referred to by name, the name should be understood as standing for the pure, strength acid, and not. for some commercial strength of acids. In the case of perchloric acid, the analyses given are on the basis of the anhydrous acid, H0104.

The following examples will serve to illustrate preferred compositions of my electrolytic solution:

Exampie 1 Preferred Specifically range preferred Per cent Per cent Sulfuric acid V 15 to 20 15 Ortbo phosphoric acid 63 to 67 63 Perchloric acid l l 0.1 to 15 5 Water 17 l Substantially the balance.

With the above compositions of bath, improved polishes are obtained on 18-8, 25-12 chromiumnickel stainless steel, straight chromium stainless steels containing from 12 to 18% chromium, nickel, Monel metal, German silver, chromel, and plain carbon steels, when these metals, or articles made therefrom, are made the anodes in the electrolytic bath, at bath temperatures from room temperature up to 100 C. and at current densities of 50 amperes per sq. ft. for one hour, more or less, depending upon the original conditions of the surface. At higher current densities, less time is required. In general, the current densities may be increased to as much as 250 amperes per sq. ft., or even higher, but where higher current densities are employed, the higher cost of electrical energy offsets the saving in time. The length of time to effect the desired result depends upon the magnitude of the current densities employed and to some extent upon the particular metal to be polished, as well as the character. of its surface initially. Rough surfaces, of course, require a longer time to polish than relatively smooth ones.

The mechanism by which the highly polished and lustrous surfaces characteristic of my invention are produced is not well understood, but is undoubtedly associated with the presence of a polarizing film over the surface of the metal during the process of anodic attack. The nature of this film is such that selective attack of the various phases present in the metal or alloy is minimized. Anodic dissolution apparently takes place at a relatively high rate at high anodic polarization value. The result of the anodic dissolution of the metal under these conditions is to level the crystal surface of the metal to produce a smooth and mirror-like finish. These conditions do not prevail in'the simple electrolytic cleaning treatment known to the prior art.

As examples of types of electrolytic solutions containing organic solvents, the following are given:

Example 2 Per cent Ethyl alcohol Perchloric acid 13 Water Substantially the lmlzmuv.

The bath of Example 2 is suitable for polishing for polishing stainless steel, nickel and low carbon steel when any of these, or articles made threfrom, is subjected to anodic treatment at current densities of around 50 amperes per sq. ft., or higher.

Example 3 Per cent Acetic acid 82 Magnesium perchlorate 9 Water It will be understood that, instead of adding perchloric acid as such, the acid or the equivalent amount of perchlorate ions may be generated or formed by reaction within the electrolytic bath. Also, where sufliciently soluble in the bath selected, metal perchlorates may be used in place of the free acid. The effect of the addition of perchloric acid or equivalent perchlorate radicals to an electrolytic bath comprising sulfurlc and phosphoric acids is noticeable in the improved luster or polish obtained on the surface of the metal undergoing treatment, as compared with the same sulfuric-phosphoric acid bath without the perchloric acid.

'It" will thus be apparent that I have provided a novel electrolytic solution for use in producing a polish on various metals, including stainless steels, nickel and its alloys, and plain carbon steels. My method of utilizing the bath compositions described herein obviates the disadvantages of mechanical polishing operations and enables the production of polished metal articles [ill having surfaces that are free from the mechanical strains, dragging and piling effects characteristic of mechanically polished surfaces.

It will, of course, be understood that various details of the process may be varied through a wide range without departing from the principles of this invention and it is, therefore, not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. An electrolytic solution for use in the anodic polishing of metals andalloys selected from the group consisting of iron and nickel and their alloys and German silver, consisting essentially of about 15% of sulfuric acid, 63% of ortho phosphoric acid and 5% of perehloric acid by weight, the balance being mainly water.

2. The method of anodically polishing metals and alloys selected from the group consisting of iron and nickel and their alloys and German silver, which comprises subjecting the metal to be polished as the anode to the action of an aqueous electrolyte consisting essentially of 15% of sulfuric acid, 63% of ortho phosphoric acid and 5% of perchloric acid by weight at a tem perature not over about 100 C., at a current density of about to 250 amp./sq. ft. and for a suflicient length of time to effect a polished surface on said metal.

CHARLES L. FAUST. 

